The present invention relates to retainers for preventing axial movement of turbine blades with respect to a turbine wheel and more specifically, to retainers that must be individually installed after all of the blades about the circumference of a turbine wheel are in place.
In turbo machinery, the turbine blades or buckets are typically attached to a turbine wheel or rotor by way of interlocking parts, commonly known as a firtree connection. The turbine blade carries at its base a male or root portion of the connection, while the wheel has at its circumference corresponding female or slot portions. A particular blade is engaged to the wheel by sliding the male portion of the fir tree of the blade into a female portion of the fir tree defined by the turbine wheel. Shoulders of the fir tree then radially secure the blade. Of course, the blades must also be axially secured.
Perhaps the most common method of axially securing the blades of a turbine involves the insertion of locking keys or pins into matching holes in the rotor and turbine blade. In typical systems each blade is configured with a platform which covers the securing pin of the adjacent turbine blade, and the last blade attached to the wheel is secured to the wheel differently from the other blades, typically by peening. Generally, in accordance with the above discussion, the blades must be attached to the wheel one-by-one in a predetermined sequence, and removed in the same manner. It is not always possible to sequentially attach the blades because of the blade geometry. For example, in turbines with vibration-damping integral interlocking shrouds, the blades of a particular turbine wheel are all connected via the shroud and assembled into a loading jig or fixture before they are loaded into the turbine wheel. It is therefore not possible to place the blades on the wheel one-by-one in a predetermined sequence since all of the blades must be placed on the wheel at the same time.
Various methods are known for axially securing turbine blades for use after all of the blades are in place on the wheel.
One method involves inserting a stainless-steel tube through a hole in the wheel, located at the junction of the blade and wheel. The hole is chamfered at both ends and after insertion, the tube is flared. Although acceptable for the service intended, the flared tube arrangement has several drawbacks. Assembly must be carried out by highly skilled craftsmen, particularly because of the chamfering and flaring operations. Excessive flaring can easily split the tube and/or the flaring operation can cause localized extrusion of tube material into the gap area between the blade and the wheel. This latter condition can cause stress concentrations leading to turbine failure.
Other known methods of axially securing turbine blades include soft metal rivets, as taught by Kurti in U.S. Pat. No. 2,753,149; bolts, as disclosed by Harris in U.S. Pat. No. 4,037,990 and by Asplund, et al. in U.S. Pat. No. 3,936,222; by way of interlocking side plates, as shown by Auriemma in U.S. Pat. No. 4,279,572; and by using a three piece blade-lock as described by Burge, et al. in U.S. Pat. No. 3,395,891.
Despite extensive efforts to develop individually installable turbine blade retainers, all such arrangements heretofore known have fallen short of achieving the desired characteristics in terms of reliability, rigidity, strength, and ease of installation.
It is accordingly an object of the invention to provide an individually installable turbine blade retainer which eliminates the need for precision installation techniques.
It is another object of the invention to provide a stronger, more rigid and secure turbine blade retainer for preventing axial motion of turbine blades in turbo machinery.
It is a further object of the invention to provide means for axially securing a turbine blade by a consistent and predictable procedure so that repairs and replacement of blades can be made by unskilled workers in a more reliable manner than heretofore known.
Still further objects and advantages of the inventive turbine blade retainer not specifically set forth here will become readily apparent upon consideration of the following description, drawings, and claims.